Turbomachines, especially gas turbines (in the broader sense), have a gas turbine (in the narrower sense) in which a hot gas, which beforehand has been compressed in a compressor and heated in a combustion chamber, is expanded to produce work. For high mass flows of the hot gas, and therefore high power ranges, gas turbines are constructed in an axial structural design, wherein the gas turbine is formed from a plurality of blade rings which are in series in the throughflow direction. The blade rings have impeller blades and diffuser blades which are arranged over their circumference, wherein the impeller blades are fastened on a rotor of the gas turbine and the diffuser blades are fastened on the casing of the gas turbine.
Such turbine blades are known from JP 206 307 842 A.
The higher the inlet temperature of the hot gas in the gas turbine is, the higher is the thermodynamic efficiency of gas turbines. However, limits are set upon the level of the inlet temperature by the thermal loadability of the turbine blades. Consequently, an aim is to create turbine blades which even in the case of high thermal loads have an adequate mechanical strength for operation of the gas turbine. To this end, turbine blades are provided with costly coating systems. For further increase of the permissible turbine inlet temperature turbine blades are cooled during operation of the gas turbine. In this case, film cooling constitutes a very effective and reliable method for cooling highly stressed turbine blades. In this, cool air is tapped from the compressor and guided into the turbine blades which are provided with internal cooling passages. After convective cooling of the materials from the inner side of the turbine blades, the air is directed onto the outer surface of the turbine blades by means of fluid passages. There, it forms a film which flows along the outer surface of the turbine blade and cools these and also protects them from the hot flow at the same time.
An ideal film cooling could be achieved with the aid of a slot blow-out system. Since this cannot be realized on turbine blades from the structural-mechanical point of view, cylindrical fluid passages or even fluid passages with an oval cross section are used in the first instance on account of manufacturability. Close to the principle of slot cooling, it is furthermore known to widen the cross section of the flow passages at their outlet, i.e. in the manner of a diffuser in their outflow passage section. In this case, the outlet cross section is increased by a determined factor. This leads to a fanning-out of the cooling air jet which, independently of the flow situation, involves a lowering of the jet impulse, lower mixing losses and a larger lateral covering. It is generally considered that contoured holes lead to an increase of effectiveness in the region of the fluid-passage longitudinal axis and overall to a better lateral covering.
Trials have shown that the cooling air in the fluid passages or cooling passages separates from their wall. As shown in FIG. 14, such a separation takes place especially in the outflow passage section of diffuser-like design of the fluid passage, specifically on its downstream wall region, as seen with regard to the flow direction of the hot gas, or wall region situated toward the cold gas side. Furthermore, trials have shown that when the fluid passages are exposed to throughflows vortex formations occur, as are shown in FIG. 15. Four different vortex structures can be identified in the main.
Annular vortices Ω1: The cooling air jet acts like an inclined cylinder upon the main flow and accelerates this. Pressure differences are formed between the side facing upstream and downstream and the upper side of the cooling air jet, which lead to a compensating flow. As a result, annular vortices Ω1 are formed. The rotation of the discharging boundary layer of the cooling air supports this effect.
Reniform vortices Ω2: The reniform vortices are a result of a vortex pair which occurs in the fluid passage. Friction forces in the free shear layer between the discharging cooling fluid jet and the main flow additionally intensify the rotation.
Horseshoe vortices Ω3: Horseshoe vortices Ω3 occur in the stagnant zone of a cylinder which is vertical in a boundary layer flow. Close to the wall, the pressure in the boundary layer is minimal. In contrast to this, in the outer layer of the main-flow boundary layer a positive pressure gradient is formed. The boundary layer separates and rolls against the wall against the main flow in the direction of the pressure minimum. The ensuing vortex is located on both sides around the cylinder. The direction of rotation of the horseshoe vortices Ω3 is opposite to that of the adjacent reniform vortices Ω2, and the horseshoe vortices Ω3 extend laterally beneath the cooling air jet during individual-hole blow-out.
Unsteady vortices Ω4: The unsteady vortices are comparable to Kármán vortices in the wake of a cylinder. The cause of the vortex formation is the boundary layer separation on the suction side of the cylinder. The unsteady vortices Ω4 occur vertically on the cooled surface.
If, therefore, hot gas from a combustion chamber of the turbomachine on the outer surface of the turbine blade meets a jet of cooling fluid discharging from the fluid passage, then the flow of hot gas is distributed around the cooling fluid jet, and a chimney vortex, with two vortex arms Ω2, is formed as a result of the action of the hot gas on the jet edge. Each of the two vortex arms Ω2 is formed by one vortex, wherein the velocity vectors of the hot gas on the two inner sides of the vortex arms point away from the outer wall.
In order to influence the vortex formation, it is known to provide turbolators in the form of fins or pins in the fluid passages (see WO 2013/089255 A1 and US 2009/0304499 A1).